Since the BC is edible, it is utilized in the food industry. The BC's high dispersibility in water further provides it with a lot of industrial utility value, such as to maintain viscosity of food, cosmetics or coating agents, to strengthen food materials, to maintain moisture, to improve stability of food, and to be used as low-calorie additives and an emulsion stabilizer.
The BC is characterized by a sectional width of its fibrils, which is smaller by two orders of magnitude than that of other kinds of cellulose such as those derived from wood pulp.
Due to such structural and physical features of microfibrils, a homogenized BC has plenty of industrial utility as a strengthening agent for polymers, especially hydrophilic polymers. Products prepared by solidification of the homogenized BC in the form of a lump or paper show a high elastic modulus in tension due to the above feature, and are therefore expected to have excellent mechanical properties for use in various kinds of industrial materials.
Methods for the production of the BC are described in, for example, Japanese Patent Laid-Open Application Sho 62(1987)-265990, Japanese Patent Laid-Open Application Sho 63(1988)-202394 and Japanese Patent Publication Hei 6(1994)-43443.
As a nutrient medium suitable for the culture of the cellulose-producing bacteria, Schramm/Hestrin medium is known, which contains carbon source, peptone, yeast extract, sodium phosphate and citric acid (Schramm et al., J. General Biology, 11, pp.123-129, 1954). Further, it has been found that the productivity of the BC is increased by the addition of an accelerator for the cellulose production such as inositol, phytic acid and pyrroloquinoline quinone (PQQ) (Japanese Patent Publication Hei 5(1993)-1718; Mitsuo TAKAI, Japan TAPPI Journal, Vol.42, No.3, pp.237-244), carboxylic acid or their salts (Japanese Patent Application Hei 5(1993)-191467; Japanese Patent Laid-Open Application Hei 7(1995)-39386), invertase (Japanese Patent Application Hei 5(1993)-331491; Japanese Patent Laid-Open Application Hei 7(1995)-184677) and methionine (Japanese Patent Application Hei 5(1993)-335764; Japanese Patent Laid-Open Application Hei 7(1995)-184675) into such a nutrient medium. Furthermore, a method for cultivating the cellulose producing bacteria under the specific range of oxygen-transfer coefficient (K.sub.L a) condition has been proposed (Japanese Patent Application Hei 7(1995)-31787).
The bacteria may be generally cultured in any known culture conditions such as static culture, shaken culture, and aerated and agitated culture, and in any known culture operation methods such as batch fermentation, fed batch fermentation, repeated batch fermentation and continuous fermentation.
Means for agitation include impellers (agitating blades), air-lift fermenters, pump-driven recirculation of the fermenter broth and any combination of these means.
The impellers include gate-shape impellers, turbine impellers, double helical ribbon impellers and screw impellers.
The productivity in the culture of bacteria has been generally improved by the increment of the inner pressure within the fermentation tank, followed by the increment of an oxygen-supply into a culture medium.
The above advantage may be contributed to the increment of a partial pressure of O.sub.2 in the gas phase (air bubbles), which will cause the increment of oxygen-transfer according to the following formula: EQU dC.sub.L /dt=K.sub.L a (C*-C.sub.L)=H K.sub.L a (P.sub.G -P.sub.L)
wherein
dC.sub.L /dt: Oxygen-transfer rate (mmol/L.hr) PA1 K.sub.L a: Oxygen-transfer coefficient (hr.sup.-1); PA1 C.sub.L : Dissolved oxygen concentration in the culture medium (mmol/L); PA1 C*: Dissolved oxygen concentration (mmol/L) which is in an equilibrium state with the partial pressure of oxygen in the air bubbles; PA1 H: Henry's constant; PA1 P.sub.G : Partial pressure of oxygen in the gas phase; and PA1 P.sub.L : Partial pressure of oxygen in the liquid phase.
However, the effect of the internal pressure within the fermentation tank on the BC production has not yet been clarified.
In the prior BC production methods, accumulation of the BC in the culture medium due to the culture of the cellulose-producing bacteria will increase the viscosity of the culture medium and make it difficult to transfer oxygen into the culture medium, so that the increment of an agitation rate or an amount of aeration was needed to fulfill the necessary amount of oxygen-supply. However, these prior methods are not economical since they need much power. Up to now, the shape of impellers and fermentation tank has been improved (Japanese Patent Application Hei 7(1995)-31787). However, it has not yet examined in detail whether or not the increment of the internal pressure within the fermentation tank may secure the necessary amount of the oxygen-supply.
The present inventors have studied an improvement in the oxygen-supply which may be easily obtained without any specialized facilities, and now found that the power required for agitation to fulfill the oxygen-supply may be remarkably reduced by the increment of the pressure under the specific conditions.
No report has been made concerning the effect of CO.sub.2 concentration on the productivity of BC during culture. The present inventors have now found that the BC production rate and yield may be increased by maintaining the partial pressure of CO.sub.2 in the gas phase within the fermentation tank below a certain level.
The present invention has been completed on the basis of the above findings.